Bonded sand cores and molds, comprising sand and binder systems, are an important part of metal casting technology. Binder materials for use with sand cores and molds may include chemical binders or clay binders. The behavior of the sand and binder system when placed in contact with molten metal is important for determining the quality of metal parts cast using the bonded sand cores and molds. Additionally, every year, foundry industry groups and companies spend millions of dollars on refractory coatings for bonded sand cores and molds. Refractory coatings have been used to aid in surface finish improvements, and to reduce thermal expansion defects such as veining and un-bonded sand defects such as erosion.
Directional heating of bonded sand cores and molds generates anisotropic thermal gradients in the bonded sand composite material. When the shaped sand composite comes into contact with molten metal, the heat transferred from the molten metal to the sand composite causes thermo-chemical reactions that result in dimensional changes in the sand composite and thus the shape and size of the cores and molds. At any given temperature, these dimensional changes or thermal distortions are attributable to simultaneous changes in both the sand and the binder material.
In bonded sand systems, thermo-chemical reactions generally include the release of volatile materials, possible core strengthening reactions from secondary curing, and core weakening from pyrolysis. It is important to understand and distinguish between thermo-mechanical distortions caused by changes in the binder and thermo-mechanical distortions caused by changes in the sand base. It is also important to understand the likely behavior of the core or mold when molten metal is poured over the mold.